1. Field of the Invention
This invention relates to an objective lens system of an optical information recording/reproducing apparatus and, more particularly, to a chromatic aberration correcting lens system of an apparatus using a semiconductor laser as a light source.
2. Description of the Prior Art
Because an emitting light wavelength of a semiconductor laser used as a light source is shifted by changing of output power and/or temperature, when the chromatic aberration of the lens system is not corrected, the position of a light converging point is varied when the wavelength is shifted. But as the optical disk apparatus has a focusing servo mechanism which moves the objecting lens system along an optical axis thereof, an out of focus condition due to a comparatively gentle change of a wavelength caused by a change in temperature or the like is automatically corrected by said focusing servo when a collimator lens is corrected for chromatic aberration and change of temperature.
However, at the time when a data is written, an oscillation wavelength of the semiconductor laser is instantaneously shifted by several nm between a region where the temperature is increased and a region where the temperature is not increased. And the out of focus caused by such radical shift cannot be corrected by the above-mentioned focusing servo.
When the light converging position is not coincident with the recording surface of the disk, there is a high possibility that incorrect writing and reading will result.
Therefore, especially when writing is effected, correction of the chromatic aberration of the objective lens system is important.
An objective lens system which is corrected in the chromatic aberration is disclosed in, for example, Japanese Patent Early Laid-open Publications No. Sho 51-18557, No. Sho 57-74716, No. Sho 58-72114, No. Sho 60-232519, No. Sho 61-91612 and No. Sho 62-35310.
As the objective lens for the optical disk apparatus is moved at high frequency for the purposes of focusing and tracking, the lighter in weight of the lens system is, the smaller the burden to the actuator is and the higher the following ability is. And, it is desirable to have a great working distance between last surface of the objective lens system and the optical disk in order to drive the lens system easily.
Therefore, it is demanded that the objective lens system is long in working distance and short in focal length in order to be light in weight.
Particularly, the objective lens system of the data recording and reproducing apparatus is driven at random, and it is demanded to be light in weight compared with a so-called compact disk apparatus for reproducing only.
The lens system of the Japanese Patent Early Laid-open Publication No. Sho 51-18557 is short in the working distance, and a correction of the chromatic aberration is not enough.
The lens system of the Japanese Patent Early Laid-open Publication No. Sho 58-72114 corrects the chromatic aberration by second and third surfaces of the cemented lens which is formed of three lenses, and both surfaces of a negative lens therein are concave.
This lens system is designed such that a second lens in the cemented lens has a lower refractive index than a third lens in order to balance a correction of coma and spherical aberrations. However, this condition makes it to be difficult to select a high dispersion material for the third lens of the cemented lens. Therefore, this lens system is designed such that a focal length of hole system is determined by an outgoing surface of the third lens in order to correct the chromatic aberration, and it is for that reason the length of the lens system is so long.
Also, the cemented lens which is consists of three lenses has a high possibility that decentering has occurred. As the decentering makes aberrations change greatly, the construction of cemented three lenses is highly sensitive to change in aberrations.
The lens system of Japanese Patent Early Laid-open Publications No. Sho 60-232519 includes two combinations of cemented lens which are formed of a strong power negative lens and a positive lens. The refractive index of each positive lens is lower than that of each negative lens and the curvature of the positive lens is small in order to correct the spherical aberration by the cemented surface. Therefore, an effective aperture of the positive lens is small in order to obtain a sufficient edge thickness, and it is difficult to make a bright lens more than 0.50 in numerical aperture (NA).
Moreover, the lens system of Japanese Patent Early Laid-open Publication No. Sho 62-35310 is designed to consider not only correction of the chromatic aberration, but also corrections of a curvature of field and an astigmatism. However, in this lens system, a thickness of a second lens group as a negative meniscus lens and a distance between the second lens group and a third lens group are so long. For that reason this lens system is very long in length and heavy in weight.
Japanese Patent Early Laid-open Publications No. Sho 57-74716 discloses the lens system which aim at light weight and long working distance. However, this lens system is designed such that the distance between a second lens and a third lens is more than 0.55 f (f: focal length) in order to correct coma aberration, consequently the sum of the length of lens system and back focal distance is more than 2.8 f. Thus, the lens system is not light in weight.
On the other hand, Japanese Patent Early Laid-open Publications No. Sho 61-91612 discloses a 2-groups 3-pieces type objective lens system which is designed for a read only apparatus. NA of this lens is 0.47, and it is light in weight and long in working distance.
As the lens system of No. Sho 61-91612 is designed without considering the correction of the chromatic aberration, glass material of a third lens is selected upon its refractive index that has a high index, height dispersion and of low cost glass material. However, even if the glass material of the third lens selected has the smallest dispersion material from the glass material group which has about the same refractive index, correction of the chromatic aberration is insufficient.
Also, if this type objective lens system satisfies the condition of NA=0.55 which is demanded for the objective lens for recording, a middle portion of the spherical aberration swells owing to generation of a high order spherical aberration.